In diseased hearts having conduction defects and in congestive heart failure (CHF), cardiac depolarizations that naturally occur in one upper or lower heart chamber are not conducted in a timely fashion either within the heart chamber or to the other upper or lower heart chamber. In such cases, the right and left heart chambers do not contract in optimum synchrony with each other, and cardiac output suffers due to the conduction defects. In addition, spontaneous depolarizations of the left atrium or left ventricle occur at ectopic foci in these left heart chambers, and the natural activation sequence is grossly disturbed. In such cases, cardiac output deteriorates because the contractions of the right and left heart chambers are not synchronized sufficiently to eject blood therefrom. Furthermore, significant conduction disturbances between the right and left atria can result in left atrial flutter or fibrillation.
It has been proposed that various conduction disturbances involving both bradycardia and tachycardia of a heart chamber could benefit from pacing pulses applied at multiple electrode sites positioned in or about a single heart chamber or in the right and left heart chambers in synchrony with a depolarization which has been sensed at least one of the electrode sites. It is believed that cardiac output can be significantly improved when left and right chamber synchrony is restored, particularly in patients suffering from dilated cardiomyopathy and CHF.
A number of proposals have been advanced for providing pacing therapies to alleviate these conditions and restore synchronous depolarization and contraction of a single heart chamber or right and left, upper and lower, heart chambers as described in detail in commonly assigned U.S. Pat. Nos. 5,403,356, 5,797,970 and 5,902,324 and in U.S. Pat. Nos. 5,720,768 and 5,792,203 all incorporated herein by reference. The proposals appearing in U.S. Pat. Nos. 3,937,226, 4,088,140, 4,548,203, 4,458,677, 4,332,259 are summarized in U.S. Pat. Nos. 4,928,688 and 5,674,259, all incorporated herein by reference. The advantages of providing sensing at pace/sense electrodes located in both the right and left heart chambers is addressed in the '688 and '259 patents, as well as in U.S. Pat. Nos. 4,354,497, 5,174,289, 5,267,560, 5,514,161, and 5,584,867, also all incorporated herein by reference.
The medical literature also discloses a number of approaches of providing bi-atrial and/or bi-ventricular pacing as set forth in: Daubert et al., "Permanent Dual Atrium Pacing in Major Intra-atrial Conduction Blocks: A Four Years Experience", PACE (Vol. 16, Part II, NASPE Abstract 141, p.885, April 1993); Daubert et al., "Permanent Left Ventricular Pacing With Transvenous Leads Inserted Into The Coronary Veins", PACE (Vol. 21, Part II, pp. 239-245, January 1998); Cazeau et al., "Four Chamber Pacing in Dilated Cardiomyopathy", PACE (Vol. 17, Part II, pp. 1974-1979, November 1994); and Daubert et al., "Renewal of Permanent Left Atrial Pacing via the Coronary Sinus", PACE (Vol. 15, Part II, NASPE Abstract 255, p. 572, April 1992), all incorporated herein by reference.
Problems surface in implementing multi-site pacing in a single heart chamber or in right and left heart chamber pacing within the contexts of conventional timing and control systems for detecting sense event signals generated by sense amplifiers coupled to spaced apart pace/sense electrodes. The application of closely timed pacing pulses to the right and left heart chamber or at spaced apart sites in the same heart chamber and the detection of conducted depolarizations are complicated due to other actions that must be taken after delivery of pacing pulses to allow sense amplifiers to be reconnected to the sense/pace electrodes in as short a time as possible.
Typically, a negative-going or cathodal voltage pacing pulse is applied to a small surface area, active pace/sense electrode, which is typically the tip electrode of an endocardial lead lodged against the heart tissue. The pacing pulse is produced by the exponential discharge of an output capacitor through the impedance load in the pacing path including a coupling capacitor, the pace electrodes, and the patient's heart tissue between the pace electrodes.
Immediately following delivery of a pacing pulse to cardiac tissue, a residual post-pace polarization signal (or "after-potential") remains in the pacing path due to the residual energy in the impedance load into which the output capacitor is discharged to deliver the pacing pulse. The impedance load across the output amplifier terminals comprises the impedance of the coupling capacitor, the lead conductor(s), the tissue-electrode interface impedances, and the impedance of the body tissue bulk between the active and indifferent electrodes. The impedances of the body tissue and the lead conductor(s) may be modeled as a simple series bulk resistance, leaving the tissue-electrode interfaces and any coupling capacitors as the reactive energy absorbing/discharging elements of the total load. There are typically two tissue-electrode interfaces in a pacing path, one at the active tip electrode, and one at the indifferent ring (or IPG case or "can") electrode. The energy stored in these interfaces and any coupling capacitors dissipates after the pacing pulse through the pacing path impedance load creating the after-potential that can be sensed at each electrode and affect the ability of the sense amplifiers to sense natural or evoked cardiac events. The tip electrode is the primary after-potential storage element in comparison to the case and ring electrodes. An indifferent ring electrode typically stores more energy than does a can electrode due to differences in electrode areas.
In conventional pacing systems, the sense amplifiers are "blanked", i.e., uncoupled, from the pace/sense electrodes during the delivery of the pacing pulse and for a programmed blanking period thereafter until the repolarization of the tissue-electrode interfaces operation takes place. Most current pacemaker output amplifiers circuits incorporate "fast recharge" circuitry for short circuiting the pacing path and actively dissipating or countering after-potentials during the blanking of the sense amplifier's input terminals to shorten the time that it would otherwise take to dissipate afterpotentials. Fast recharge circuitry and operations are described in commonly assigned U.S. Pat. Nos. 4,406, 286, 5,782,880, and German OLS DE 196 15 159, all incorporated herein by reference. The primary purposes of providing a recharge operation are to ensure that the coupling capacitor(s) is recharged to an insignificant voltage level or equilibrium prior to the delivery of the next pacing pulse through it and to allow the net DC current in the pacing path to settle to zero to facilitate sensing in the same pacing path or using one of the pace/sense electrodes of that pacing path.
In the case of bi-chamber pacing of the type described in the above-incorporated '324 patent, for example, it is desirable to be able to deliver first and second pacing pulses through independent output amplifier circuits in close spacing, which has been suggested to be as short as 0 msec to 80 msec. We have discovered that the pacing discharge and recharge circuits for each such pacing pulse can overlap one another and interfere with coupled pacing under certain circumstances. This interference can prevent the delivery of simultaneous pacing pulses and limit the minimum delay between the delivery of the right and left chamber pacing pulses to the recharge time for the first delivered pacing pulse.
Similar problems arise in delivering closely spaced pacing pulses to multiple sites in the same heart chamber and recharging the multiple pacing pathways. Moreover, these problems arise in providing multiple, closely timed, electrical stimulation pulses to living tissue through multiple reactive stimulation paths where recharging the stimulation paths is necessary to alleviate polarization after potentials prior to delivery of subsequent stimulation pulses to the same stimulation paths.